Existing railway vehicles include a first and a second electrical converter each capable of transforming a single phase or direct electric voltage received via a catenary into a three-phase electrical voltage delivered to a first and second three-phase power supply network respectively on board, each capable of distributing electrical power to several cars of the railway vehicle, the first and second networks being electrically isolatable from one another, several air-conditioning units installed in several cars respectively, each air-conditioning unit comprising at least one compressor to produce cold air which is expelled into the interior of the car, this compressor being electrically connected to the first network in order to receive three-phase voltage power from the first converter, and auxiliary electrical loads other than the compressors electrically connected to the second power supply network to be powered with three-phase voltage by the second converter.
The term “catenary” is used to mean both a conducting wire suspended in air along which a pantograph slides to provide power to the railway vehicle and an additional rail laid on the ground against which a shoe presses in order to power the railway vehicle. This latter technology is known by the term “third rail”.
Here “cold air” means air whose temperature is less than the ambient temperature within a car.
Conventionally, in order to balance the loads on the first and second converters, half of the compressors of the air-conditioning units are electrically connected to the first converter and the other half are connected to the second converter. The auxiliary loads are distributed between the first and second converters for the same reason.
Because the auxiliary loads need to be powered by a voltage and a frequency generally fixed between 50 Hz and 60 Hz, each of the first and second converters deliver this fixed frequency alternating voltage to their corresponding systems. In this situation the compressors of each air-conditioning unit are also powered by a fixed voltage and frequency and each air-conditioning unit therefore has the same electrical power to power its respective compressor or compressors.
In modern air-conditioning units the compressor systematically takes up the maximum power available on the electrical system to which it is connected in order to ensure a high level of air-conditioning comfort for the passengers in the car.
A mechanism for adjusting the temperature of the cold air expelled for the same maximum electrical power available on the first system therefore has to be provided in each air-conditioning unit. Some adjustment mechanisms in fact only reduce the energy efficiency of the air-conditioning unit to a greater or lesser extent. For example, when electrical power P is taken up by the compressor, the temperature of the cold air will be X degrees if the energy efficiency is Δ and the temperature of the cold air will be above X degrees if the energy efficiency is less than Δ.